Hi Nick Testing and comparing BC custom blend modes and Photoshop/Pixelmator ones, I've noticed that layers in IT seem to be reversed: what would be at top (blend layer) seems to be at bottom (base layer) and the opposite of course.This is easy to verify, using Color Blend Mode and Luminosity Blend Mode (each one is the opposite of the other one).You may compare IT with Pixelmator, Photoshop and/or Fun House (Apple Developer Tools).You may also test Overlay and Hard Light blend modes.

I have compared IT, AT and Photoshop. On my opinion, in IT layers aren't swapped. "Soft Light Blend Mode" was used for tests. I've made sure that the test images give different results depending on their position (top or bottom layer). In IT, the main image was considered as the top layer, and the one I selected in the Composite Filter options was the bottom layer (Background Image).

The only thing I have noticed is that some colors were blended in different way if we compare Photoshop with Core Image filters in IT and AT.

Hi NickHere's the result of an interesting test comparing Color Blend Mode and Luminosity Blend Mode. It's interesting because swapping layers with one of these modes creates the same result than if you would have been used the other mode.Using Color Blend Mode then swapping the two images is the same than using Luminosity one.Here's what I got when opening the castle image in IT then adding the blue gradient image as the Background Image (seems like a very ambigious name from my point of view and would be named something like "blend image" or "texture image"):

I agree with you that the interface is a bit confusing. In Photoshop, the background layer is frequently the "main" image you need to modify, and the upper layer(s) are intermediate (masks, gradients, etc.). I'll offer the programmers to swap them in the interface.

The Wikipedia gives this definition for Color and Luminosity blend modes:• The Color blend mode preserves the luma of the bottom layer, while adopting the hue and chroma of the top layer.• The Luminosity blend mode preserves the hue and chroma of the bottom layer, while adopting the luma of the top layer.

Similar results for Color and Luminosity blending applied to the same pair of images stacked in different orders seems to be explainable. But I never knew about existence of this effect.